I just discovered that if a user goes directly to the link with the form, they get the 403 message. If one first visites other pages on the website, the form gives no errors. I've read that CSRF blocks the form submission if the previous page is not from the same website. This means that I can never sent the subscribtion link directly to people.
How could this be solved? Disable CSRF is no option because of the spam.
Regards Niels